Method and apparatus for electrochemical compression and expansion of hydrogen in a fuel cell system

ABSTRACT

The invention relates to fuel cell systems and associated methods of operation where an electrochemical cell such as a fuel cell is used as an electrochemical hydrogen separator to separate hydrogen from a process stream (e.g., reformate or synthesis gas), or as an electrochemical hydrogen expander to inject hydrogen into a process stream. In one aspect, the invention provides a method of operating a fuel cell system, including the following steps: flowing hydrogen from a hydrogen supply conduit through a fuel cell to provide an electric current to a load coupled to the fuel cell; actuating an electrochemical hydrogen separator in a first mode of operation of the system to transfer hydrogen from the hydrogen supply conduit to a hydrogen storage vessel; and actuating an electrochemical hydrogen expander in a second mode of operation of the system to transfer hydrogen from the hydrogen storage vessel to the fuel cell.

BACKGROUND

[0001] The invention relates to fuel cell systems and associated methodsof operation where an electrochemical cell such as a fuel cell is usedas an electrochemical hydrogen separator to separate hydrogen from aprocess stream, or as an electrochemical hydrogen expander to injecthydrogen into a process stream.

[0002] A fuel cell is an electrochemical device that converts chemicalenergy produced by a reaction directly into electrical energy. Forexample, one type of fuel cell includes a polymer electrolyte membrane(PEM), often called a proton exchange membrane, that permits onlyprotons to pass between an anode and a cathode of the fuel cell. At theanode, diatomic hydrogen (a fuel) is reacted to produce protons thatpass through the PEM. The electrons produced by this reaction travelthrough circuitry that is external to the fuel cell to form anelectrical current. At the cathode, oxygen is reduced and reacts withthe protons to form water. The anodic and cathodic reactions aredescribed by the following equations:

H₂→2H⁺+2e⁻  (1)

[0003] at the anode of the cell, and

O₂+4H⁺+4e⁻→2H₂O  (2)

[0004] at the cathode of the cell.

[0005] A typical fuel cell has a terminal voltage of up to about onevolt DC. For purposes of producing much larger voltages, multiple fuelcells may be assembled together to form an arrangement called a fuelcell stack, an arrangement in which the fuel cells are electricallycoupled together in series to form a larger DC voltage (a voltage near100 volts DC, for example) and to provide more power.

[0006] The fuel cell stack may include flow field plates (graphitecomposite or metal plates, as examples) that are stacked one on top ofthe other. The plates may include various surface flow field channelsand orifices to, as examples, route the reactants and products throughthe fuel cell stack. A PEM is sandwiched between each anode and cathodeflow field plate. Electrically conductive gas diffusion layers (GDLs)may be located on each side of each PEM to act as a gas diffusion mediaand in some cases to provide a support for the fuel cell catalysts. Inthis manner, reactant gases from each side of the PEM may pass along theflow field channels and diffuse through the GDLs to reach the PEM. ThePEM and its adjacent pair of catalyst layers are often referred to as amembrane electrode assembly (MEA). An MEA sandwiched by adjacent GDLlayers is often referred to as a membrane electrode unit (MEU).

[0007] A fuel cell system may include a fuel processor that converts ahydrocarbon (natural gas or propane, as examples) into a fuel flow forthe fuel cell stack. For a given output power of the fuel cell stack,the fuel flow to the stack must satisfy the appropriate stoichiometricratios governed by the equations listed above. Thus, a controller of thefuel cell system may monitor the output power of the stack and based onthe monitored output power, estimate the fuel flow to satisfy theappropriate stoichiometric ratios. In this manner, the controllerregulates the fuel processor to produce this flow, and in response tothe controller detecting a change in the output power, the controllerestimates a new rate of fuel flow and controls the fuel processoraccordingly.

[0008] A fuel cell may also operate on pure hydrogen or other streamscontaining hydrogen such as synthesis gas. In some cases, especially atoperating temperatures lower than 100° C., carbon monoxide may poisonfuel cell catalysts (e.g., platinum), so it may be desired to removecarbon monoxide from such a stream (e.g., to less than 50 parts permillion) before flowing it through a fuel cell. Some fuel cell membranematerials such as the polybenzimidazole membrane available from Celaneseoperate at higher temperatures (e.g., greater than 120° C.), so thatcarbon monoxide is less of a concern in this respect. Direct methanolfuel cells are also known that can react methanol directly to achieveproton exchange through the fuel cell membrane.

[0009] The fuel cell system may provide power to a load, such as a loadthat is formed from residential appliances and electrical devices thatmay be selectively turned on and off to vary the power that is demandedby the load. Thus, the load may not be constant, but rather the powerthat is consumed by the load may vary over time and abruptly change insteps. For example, if the fuel cell system provides power to a house,different appliances/electrical devices of the house may be turned onand off at different times to cause the load to vary in a stepwisefashion over time. Fuel cell systems adapted to accommodate variableloads are sometimes referred to as “load following” systems.

[0010] There is a continuing need for integrated fuel cell systems andassociated process designed to achieve objectives including the forgoingin a robust, cost-effective manner.

SUMMARY

[0011] In one aspect, the invention provides a fuel cell system with anelectrochemical transfer cell. As functionally defined herein, anelectrochemical transfer cell refers to an electrochemical cell that canbe used as an electrochemical hydrogen separator, or an electrochemicalhydrogen expander, or both. An electrochemical cell in this contextrefers to a device that has two electrodes sandwiching a proton orelectron conductive membrane (e.g., a PEM fuel cell). An electrochemicalhydrogen separator in this context refers to an electrochemical cellthat is used to electrochemically separate hydrogen from a fluid such asreformate. An example could be a fuel cell stack with no externalelectrical load onto which a voltage is provided. For illustrationpurposes, U.S. Pat. No. 6,280,865 is hereby incorporated by reference inits entirety. An electrochemical hydrogen expander in this contextrefers to an electrochemical cell that is used to pump hydrogen from ahydrogen concentrated electrode to an electrode less concentrated withhydrogen. For illustration purposes, U.S. Patent Ser. No. 09/540,673(currently pending) is hereby incorporated by reference in its entirety.

[0012] In one embodiment, a fuel cell, a hydrogen flow circuit (e.g., ahydrogen supply conduit leading to the fuel cell or a hydrogen exhaustconduit from the fuel cell), an electrochemical transfer cell, and ahydrogen storage vessel (e.g., a pressure tank or any enclosure or flowcircuit adapted to buffer a flow of hydrogen) are provided in thesystem. The fuel cell is adapted to receive hydrogen from the hydrogenflow circuit. The hydrogen storage vessel is coupled in fluidcommunication to the electrochemical transfer cell. In the context ofthis invention, the term “coupled” refers in a generic sense to anyconnection, either direct or indirect (e.g., in the case of an indirectconnections, other devices may be positioned between the devices thatare eventually “coupled” in a functional relationship). Theelectrochemical transfer cell is adapted to transfer hydrogen from thehydrogen flow circuit to the hydrogen storage vessel when an electricalpotential is placed on the cell (e.g., by connecting a power supply suchas a battery or a fuel cell to the electrodes).

[0013] The electrochemical transfer cell is adapted to transfer hydrogenfrom the hydrogen storage vessel to the hydrogen flow circuit when anelectrical load is placed on the cell. An electrical load may refer toany electrical connection to which electrical current may be flowed.

[0014] The system has a first operating mode in which theelectrochemical transfer cell transfers hydrogen from the hydrogen flowcircuit to the hydrogen storage vessel. The system has a secondoperating mode in which the electrochemical transfer cell transfershydrogen from the hydrogen storage vessel to the hydrogen flow circuit.

[0015] In some embodiments, the system further includes a fuel processor(also referred to as a reformer, without respect to the reactionemployed) coupled to the hydrogen flow circuit and adapted to supplyreformate to the hydrogen flow circuit.

[0016] In some embodiments, the system further includes a compressorcoupled between the electrochemical transfer cell and the hydrogenstorage vessel and adapted to pressurize the hydrogen storage vessel. Avessel outlet conduit may also be provided between the hydrogen storagevessel and the electrochemical transfer cell with a pressure regulatorlocated along the vessel outlet conduit to regulate a pressure ofhydrogen supplied to the electrochemical transfer cell from the hydrogenstorage vessel. As examples, the compressor can have an outlet pressureof greater than 1000 psia, or greater than 3000 psia. In some cases, itmay be desirable to operate the storage vessel as pressures around 5000psia. In embodiments where the transfer cell can withstand suchpressures, it may be desirable to eliminate the compressor and rely onthe transfer cell to provide the pressure to the storage vessel.

[0017] In some embodiments, the fuel cell is a stationary fuel cell,such as one of the residential PEM systems manufactured by Plug Power,Inc. The system can further include an outlet valve coupled to thehydrogen storage vessel to allow hydrogen to be transferred from thehydrogen storage vessel to a secondary hydrogen storage vessel, such asa hydrogen tank associated with a hydrogen powered vehicle.

[0018] In some embodiments, a system controller can be provided toswitch the system between the first and second operating modes describedabove. As an example, an electrical load may be connected to the fuelcell and monitored by the controller (e.g., by monitoring the poweroutput and voltage effects on the fuel cell) such that the controllerswitches the system to the first mode when the load is below a firstpredetermined threshold (e.g., a fuel cell in a fuel cell stack fallsbelow a threshold such as 0.6 volts), and the controller switches thesystem to the second mode when the load is above a second predeterminedthreshold (e.g., a fuel cell in a fuel cell stack with the lowestvoltage is above a threshold such as 0.8).

[0019] This determination generally relates to the stoichiomteric ratioof hydrogen provided to the fuel cell with respect to the amountdemanded by the electrical load, and may beconducted according tovarious methods. For example, the magnitude of the load may be measuredor calculated in terms of power and compared to the power output of thefuel cell. Alternatively, the excess hydrogen from the fuel cell can bemeasured to determine whether hydrogen in present in an amount exceedingwhat is desired (e.g., 125% of the theoretical amount necessary tosatisfy the load). Alternatively, a minimum cell voltage of the fuelcell stack may be monitored to determined when the voltage falls below athreshold (e.g., 0.7 volts), indicating that additional hydrogen isneeded by the fuel cell. A lookup table may be used to correlateoperating conditions to load conditions. Other methods are known in theart for measuring or calculating whether a load on a fuel cell is aboveor below a desired threshold, and the parameters associated with the“thresholds” monitored under such embodiments of the present inventionare accordingly associated with the parameters used by such methods.

[0020] In some embodiments, an outlet of the electrochemical transfercell is coupled to the reformer and adapted to supply hydrogen to thereformer. For example, the hydrogen can be used to increase the amountof hydrogen in the reformate. In other embodiments, an outlet of theelectrochemical transfer cell can be coupled to the hydrogen supplyconduit, which can also be referred to as an inlet conduit of the fuelcell.

[0021] Systems and methods under the present invention may also be usedin combined heat and power (CHP) systems. For example, the system mayinclude a coolant circuit adapted to transfer heat from the system to aheat sink. Heat may be removed during operation from the fuel cell, thereformer, an exhaust gas oxidizer, or other components that may bepresent. A heat sink may include, as examples, a industrial and potablehot water tanks, heat exchangers for external applications, air heatingsystems for homes or buildings, etc.

[0022] In some embodiments, an oxidant flow circuit (e.g., air oroxygen) may be selectively coupled to the electrochemical transfer cell.The term “flow circuit” generically refers to an aggregate flow path ofthe through the system (e.g., including a blower, a conduit leading fromthe blower to a device inlet, a path through the device, and a pathexhausted from the device, etc.). The term “selectively coupled”indicates that the flow from the oxidant circuit to the transfer cellcan be opened or closed as desired (e.g., through manual adjustment orby a system automation controller operating a valve, etc.). In suchsystems, a third mode of operation may be provided in which anelectrical load is placed on the electrochemical transfer cell, oxidantis flowed through the oxidant flow circuit along a first electrode ofthe electrochemical transfer cell, and hydrogen is flowed through thehydrogen flow circuit along a second electrode of the electrochemicaltransfer cell, such that electrical current is supplied from theelectrochemical transfer cell to the electrical load. Theelectrochemical transfer cell can thus be used as a fuel cell togenerate power (e.g., to supplement the power generation of the mainfuel cell).

[0023] In this example, the first electrode refers to the electrode ofthe cell that receives concentrated hydrogen when the cell is used as anelectrochemical hydrogen separator. The second electrode refers to theelectrode from which hydrogen is pumped when the cell is used as anelectrochemical hydrogen separator. It is generally preferred, however,for the oxidant to flow along the second electrode and for the hydrogenfrom the storage vessel to be contacted with the first electrode. Onereason for this general preference is that in some cases (e.g., wherereformate is used), the oxidant can be flowed through the secondelectrode chamber of the cell without needing to purge the chamber ofhydrogen to avoid exothermic reaction of oxygen and hydrogen that coulddamage the cell. In other cases, depending on the gas concentrations ofthe oxidant stream and of the electrochemical cell, it may be desirableto purge the cell (e.g., with an inert gas such as nitrogen) prior toinjecting the oxidant.

[0024] In some embodiments, it is generally preferable to remove watervapor from the hydrogen stored in the hydrogen vessel to avoid freezingor corrosion problems. As an example, it may be desirable to lower thedew point of the hydrogen in the vessel to less than 30° C. below zero.In some embodiments, this may be accomplished by flowing the hydrogenacross a desiccant material, such as a coupled between theelectrochemical transfer cell and the hydrogen storage vessel. Examplesof suitable desiccant materials include molecular sieves, silica gels,clays, and blends of these materials. Such materials include, forexample, X- or Y-type zeolites (available from Linde Division of UOP),silica gels (available from Davison Division of W. R. Grace), indicatingsilica gels (available from IMPAK Corp.), Montmorillonite clays(available from IMPAK Corp.), calcium oxide, and calcium sulfate.

[0025] In some embodiments, two parallel conduits can be providedbetween the electrochemical transfer cell and the hydrogen storagevessel. For example, when one desiccant material in one conduit becomessaturated, the second desiccant material in the second conduit can beused. Where the transfer cell or fuel cell relies on polymer electrolytemembranes that require humidification, it may be desirable to humidifythe hydrogen provided from the storage vessel to fuel cell, or thetransfer cell when it is used as a fuel cell. In some embodiments,humidification can be accomplished by flowing the hydrogen from thestorage vessel across a saturated desiccant material (heated ifnecessary). It may be desirable to size the alternating paralleldesiccant beds such that the cycle frequency is less than the electricalpower demand cycle on the fuel cell system to which they are connected.

[0026] In another aspect, the invention provides a fuel cell systemincluding a first fuel cell having a hydrogen flow circuit and anoxidant flow circuit, wherein the hydrogen flow circuit comprises ahydrogen supply conduit coupled to an anode chamber of the first fuelcell, wherein the anode chamber is further coupled to a hydrogen exhaustconduit, and wherein the anode chamber comprises an anode. The oxidantflow circuit comprises an cathode supply conduit coupled to a cathodechamber of the first fuel cell, wherein the cathode chamber is furthercoupled to a cathode exhaust conduit, and wherein the cathode chambercomprises a cathode. Various embodiments may include any of the featuresor aspects described herein.

[0027] A first electrical circuit selectively provides an electricalconnection from a power supply to the first fuel cell, such that anelectrical potential is formed between the anode and cathode when thefirst circuit is activated. A second electrical circuit selectivelyprovides an electrical connection from an electrical load to the firstfuel cell, such that electrical current is drawn from the first fuelcell to the electrical load when the second circuit is activated. Ahydrogen storage vessel is coupled through a first valve to the cathodeexhaust conduit. A third electrical circuit is coupled to the firstvalve, the third circuit being adapted to open the first valve wheneither of the first or second circuits are activated, and the thirdcircuit being adapted to close the first valve when neither of the firstor second circuits are activated. Various embodiments may include any ofthe features or aspects described herein.

[0028] In another aspect, the invention provides a fuel cell systemwherein a fuel cell is coupled to a hydrogen conduit. An electrochemicalhydrogen separator is coupled to the hydrogen conduit, the separatorbeing further coupled to a hydrogen storage vessel, the separator beingadapted to selectively transfer hydrogen from the hydrogen conduit tothe hydrogen storage vessel. An electrochemical hydrogen expander iscoupled to the hydrogen conduit, the expander being further coupled tothe hydrogen storage vessel, and the expander being adapted toselectively transfer hydrogen from the hydrogen storage vessel to thehydrogen conduit. Various embodiments may include any of the features oraspects described herein.

[0029] In another aspect, the invention provides a fuel cell system,wherein a fuel cell is coupled to a hydrogen supply conduit and ahydrogen exhaust conduit. An electrochemical hydrogen separator iscoupled to the hydrogen exhaust conduit, the separator being furthercoupled to a hydrogen storage vessel. The separator is adapted toselectively transfer hydrogen from the hydrogen exhaust conduit to thehydrogen storage vessel. An electrochemical hydrogen expander is coupledto the hydrogen supply conduit, the expander being further coupled tothe hydrogen storage vessel, and the expander being adapted toselectively transfer hydrogen from the hydrogen storage vessel to thehydrogen supply conduit. Various embodiments may include any of thefeatures or aspects described herein.

[0030] In another aspect, the invention provides a fuel cell systemincluding a fuel cell, a reformer, a hydrogen flow circuit, a hydrogenstorage vessel, and an electrochemical hydrogen expander. The reformeris coupled to the hydrogen flow circuit and adapted to provide reformateto the hydrogen flow circuit. The fuel cell and the electrochemicalhydrogen expander are each coupled to the hydrogen flow circuit, whereinthe electrochemical hydrogen expander is further coupled to the hydrogenstorage vessel. An electrical circuit is coupled to the electrochemicalhydrogen expander, the circuit being adapted to selectively draw anelectrical current from the electrochemical hydrogen expander. Theelectrochemical hydrogen expander is adapted to transfer hydrogen fromthe hydrogen storage vessel to the hydrogen flow circuit in proportionto the current that is drawn by the electrical circuit from theelectrochemical hydrogen expander. Various embodiments may include anyof the features or aspects described herein.

[0031] In another aspect, the invention provides a method of operating afuel cell system, including at least the following steps: (1) placing anelectrical potential across a first fuel cell in a first mode ofoperation to transfer hydrogen from an anode chamber of the first fuelcell to a cathode chamber of the first fuel cell; (2) placing anelectrical load across the first fuel cell in a second mode of operationto transfer hydrogen from the cathode chamber of the first fuel cell tothe anode chamber of the first fuel cell; and (3) flowing air throughthe anode chamber of the first fuel cell and hydrogen through thecathode chamber of the first fuel cell in a third mode of operation toprovide an electric current to a load coupled to the first fuel cell.

[0032] In one embodiment, an additional step includes supplyingreformate from a reforming reactor to the anode chamber. In anotherembodiment, an additional step may include operating a compressor in thefirst mode of operation to pressurize a hydrogen storage vessel with thehydrogen transferred from the anode chamber to the cathode chamber. Inanother embodiment, and additional step may include: (1) transferringhydrogen in the first mode of operation from the cathode chamber to ahydrogen storage vessel; and (2) transferring hydrogen from the hydrogenstorage vessel to a secondary hydrogen storage vessel. As an example, aspreviously indicated, the secondary hydrogen storage vessel is a portionof a vehicle propulsion system.

[0033] In another embodiment, an additional step includes: (1)monitoring an electrical load on a second fuel cell; (2) switching thesystem to the first mode of operation when the load is below a firstpredetermined threshold; and (3) switching the system to the second modeof operation when the load is above a second predetermined threshold.

[0034] In other embodiments, additional steps may include flowinghydrogen from the anode chamber of the first fuel cell to a reformingreactor in the second mode of operation, or flowing hydrogen from theanode chamber of the first fuel cell to a second fuel cell in the secondmode of operation. In still other embodiments, additional steps mayinclude flowing the hydrogen from the cathode chamber of the first fuelcell across a desiccant, or humidifying the hydrogen in the cathodechamber of the first fuel cell, or flowing the hydrogen from the anodechamber of the first fuel cell through a humidifier.

[0035] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: flowing hydrogen from ahydrogen supply conduit through a fuel cell to provide an electriccurrent to a load coupled to the fuel cell; actuating an electrochemicalhydrogen separator in a first mode of operation of the system totransfer hydrogen from the hydrogen supply conduit to a hydrogen storagevessel; and actuating an electrochemical hydrogen expander in a secondmode of operation of the system to transfer hydrogen from the hydrogenstorage vessel to the fuel cell. Various embodiments may include any ofthe features, aspects, or additional steps described herein.

[0036] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) flowing hydrogenfrom a hydrogen supply conduit through a fuel cell to provide anelectric current to a load coupled to the fuel cell; (2) exhaustingunreacted hydrogen from the fuel cell to a hydrogen exhaust conduit; (3)actuating an electrochemical hydrogen separator in a first mode ofoperation of the system to transfer hydrogen from the hydrogen exhaustconduit to a hydrogen storage vessel; and (4) actuating anelectrochemical hydrogen expander in a second mode of operation of thesystem to transfer hydrogen from the hydrogen storage vessel to the fuelcell. Various embodiments may include any of the features, aspects, oradditional steps described herein.

[0037] In another aspect, the invention provides a fuel cell systemcomprising a fuel cell coupled to a hydrogen conduit, and anelectrochemical hydrogen separator coupled to the hydrogen conduit, theseparator being further coupled to a hydrogen storage vessel, theseparator being adapted to selectively transfer hydrogen from thehydrogen conduit to the hydrogen storage vessel. An outlet valve iscoupled to the hydrogen storage vessel, the outlet valve being adaptedto transfer hydrogen from the hydrogen storage vessel to a secondaryhydrogen storage vessel. As an example, the secondary hydrogen storagevessel can be a portion of a vehicle propulsion system.

[0038] In another aspect, the invention provides a fuel cell system thatutilizes a gas stream containing hydrogen and carbon monoxide (e.g.,reformate or synthesis gas). An electrochemical hydrogen separator iscoupled to the gas stream. Preferably, the electrochemical hydrogenseparator uses a PBI ion exchange membrane so that there is not aconcern with the carbon monoxide from the gas stream poisoning theseparator cell. In other embodiments, a shift reactor or preferentialoxidation stage may be provided as known in the art to remove the carbonmonoxide.

[0039] A power supply coupled to the electrochemical hydrogen separatorprovides current as discussed herein, that is effective to separatehydrogen from the gas stream on one side (electrode) of the separator,to form pure hydrogen on another side (electrode) of the separator. Thehydrogen from the separator is then utilized by a PEM fuel cell. In thisway, reformate or synthesis gas can be used to power a PEM fuel cellwithout the concern of the carbon monoxide in the gas affecting the fuelcell. Another advantage of the PBI separator is that it does not requirehumidification like other types of PEM separator systems. It may thus bedesirable to humidify the hydrogen in a humidifier (e.g., steaminjection or flowing across a water transport membrane or through anenthalpy wheel) to humidify the hydrogen prior to injecting it into thefuel cell. In some cases, for example where fuel cell lifetime is lessof a concern, it may be acceptable to utilize the hydrogen withouthumidification.

[0040] The hydrogen may also be sent to a hydrogen storage vessel. Thepower used by the separator can be provided, as examples, by a batteryor by the fuel cell. As in previous examples, one design aspect that maybe included is an outlet valve coupled to the hydrogen storage vessel,where the outlet valve is adapted to transfer hydrogen from the hydrogenstorage vessel to a secondary hydrogen storage vessel. As an example,the secondary hydrogen storage vessel can be a portion of a vehiclepropulsion system. Thus, one system provided by the invention includes astationary fuel cell system that purifies hydrogen from reformate tooperate a fuel cell to power a house or building and to charge ahydrogen storage tank for other applications such as fueling a hydrogenpowered vehicle.

[0041] Advantages and other features of the invention will becomeapparent from the following description, drawing and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a cross-sectional view of a fuel cell;

[0043]FIG. 2 is a cross-sectional view of an electrochemical transfercell that may be used as an electrochemical hydrogen separator, or as anelectrochemical hydrogen expander;

[0044]FIG. 3 is a schematic diagram of a portion of the electrochemicaltransfer cell of FIG. 2, operated in a first mode;

[0045]FIG. 3a is a schematic diagram of a portion of the electrochemicaltransfer cell of FIG. 2, operated in a second mode;

[0046]FIG. 4 is a schematic diagram of a fuel cell system with anelectrochemical transfer cell;

[0047]FIG. 5 is a schematic diagram of a fuel cell system with anelectrochemical transfer cell operated in a hydrogen separator mode;

[0048]FIG. 6 is a schematic diagram of a fuel cell system with anelectrochemical transfer cell operated in a hydrogen expander mode;

[0049]FIG. 7 is a schematic diagram of a fuel cell system with anelectrochemical transfer cell operated in a hydrogen separator mode;

[0050]FIG. 8 is a schematic diagram of a fuel cell system with anelectrochemical transfer cell operated in a hydrogen expander mode.

DETAILED DESCRIPTION

[0051]FIG. 1 shows a cross-sectional view of a fuel cell 200. Fuel cell200 includes a cathode flow field plate 210, an anode flow field plate220, a solid electrolyte 230, catalysts 240 and 250 and gas diffusionlayers 260 and 270. In the context of this invention, the terms cathodeand anode are generally used in a relative sense merely to differentiateone electrode from another in an electrochemical cell. In PEM fuelcells, generally the cathode refers to the electrode that receivesoxidant, and the anode refers to the electrode that receives hydrogen.In a fuel cell operating with such a configuration, the cathodegenerally has a positive polarity with respect to the anode. Therefore,cathode may also refer to a positively charged electrode, whereas anodemay refer to a negatively charged electrode. However, under the presentinvention, the function and polarity of a given electrode may be changedin various operating modes, so that the terms cathode and anode may notalways be descriptive of the polarity or function of an electrode at agiven time (e.g., if the terms are used to refer to the electrode in its“standard” operating mode).

[0052] Electrolyte 230 should be capable of allowing ions to flowtherethrough while providing a substantial resistance to the flow ofelectrons. Electrolyte 230 is a solid polymer (e.g., a solid polymer ionexchange membrane), such as a solid polymer proton exchange membrane(e.g., a solid polymer containing sulfonic acid groups). Such membranesare commercially available from E. I. DuPont de Nemours Company(Wilmington, Del.) under the trademark NAFION. Alternatively,electrolyte 230 can also be prepared from the commercial productGORE-SELECT, available from W. L. Gore & Associates (Elkton, Md.). Othersuitable membrane materials include the polybenzimidazole (PBI) membraneavailable from Celanese, and polyether ether ketone (PEEK) membranes.

[0053] Catalyst 240 can be formed of a material capable of interactingwith hydrogen to form protons and electrons. Examples of such materialsinclude, for example, platinum, platinum alloys, and platinum dispersedon carbon black. Alternatively, the suspension is applied to thesurfaces of gas diffusion layers 260 and 270 that face catalysts 240 and250, respectively, and the suspension is then dried. The method ofpreparing catalyst 240 may further include the use of heat, pressure andtemperature to achieve bonding. In some cases, catalyst 240 may includea mixture or alloy of platinum and ruthenium as known in the art toprevent the poisoning of the platinum by carbon monoxide that may bepresent in the anode fuel stream flowed through anode plate 220.

[0054] Catalyst 250 can be formed of a material capable of interactingwith oxygen, electrons and protons to form water. Examples of suchmaterials include, for example, platinum, platinum alloys, and noblemetals dispersed on carbon black. Catalyst 250 can be prepared asdescribed above with respect to catalyst 240.

[0055] Gas diffusion layers 260 and 270 are generally formed of amaterial that is both gas and liquid permeable material so that thereactant gases (e.g., hydrogen and oxygen) and products (e.g., water)can pass therethrough. For example, layers 260 and 270 may be a carbonfiber paper or cloth, and in some cases may be treated with ahydrophobic material such as Teflon™ to enhance water transport. Inaddition, layers 260 and 270 should be electrically conductive so thatelectrons can flow from catalysts 240 and 250 to flow field plates 220and 210, respectively.

[0056]FIG. 2 shows an embodiment of an electrochemical transfer cell 60.In this example, the transfer cell 60 can share the same components asfuel cell 200. Other configurations are possible. Electrochemicaltransfer cell 60 includes a first flow field plate 62, a second flowfield plate 64, an electrolyte 66, catalysts 68 and 70 and gas diffusionlayers 72 and 74. Electrolyte 66 should be capable of allowing ions toflow therethrough while providing a substantial resistance to the flowof electrons. Electrolyte 66 is a solid polymer (e.g., a solid polymerion exchange membrane), such as a solid polymer proton exchange membrane(e.g., a solid polymer containing sulfonic acid groups). Such membranesare commercially available from E. I. DuPont de Nemours Company(Wilmington, Del.) under the trademark NAFION. Alternatively,electrolyte 66 can also be prepared from the commercial productGORE-SELECT, available from W. L. Gore & Associates (Elkton, Md.). Othersuitable membrane materials include the polybenzimidazole (PBI) membraneavailable from Celanese, and polyether ether ketone (PEEK) membranes.

[0057] Catalysts 68 and 70 can be formed of a material capable ofinteracting with hydrogen to form protons and electrons. Examples ofsuch materials include, for example, platinum, platinum alloys, andplatinum dispersed on carbon black. Catalyst layers 68 and 70 may beformed onto electrolyte 66. Alternatively, catalyst layers 68 and 70 maybe applied to the surfaces of gas diffusion layers 72 and 74.

[0058] Gas diffusion layers 72 and 74 may be formed of a material thatis both gas and liquid permeable material so that the fuel gas and anywater condensing from the fuel gas or entrained therein can pass throughthe gas diffusion layers 72 and 74. Layers 72 and 74 should beelectrically conductive so that electrons can flow from catalysts 68 and70 to flow field plates 62 and 64, respectively. In some embodiments,the gas diffusion layers maybe omitted. In such cases, an electricalcircuit such as a power source, or an electric load may be connecteddirectly to either side of the membrane electrode assembly.

[0059] As previously discussed, an MEA refers to the sandwich of theelectrolyte 66 within the catalyst layers 68 and 70. An MEA may be usedwith or without gas diffusion layers 72 and 74. Also, it will beappreciated that flow plates 62 and 64 are also not required features ofa hydrogen pumping device. Other configurations are possible. Forexample, under the present invention it maybe desirable for the cellassembly 60 to be able to withstand high pressures (e.g., greater than1000 psia, or greater than 3000 psia).

[0060] Referring to FIG. 3, a schematic diagram is shown of a portion ofthe electrochemical transfer cell 60 of FIG. 2. In this example, theelectrochemical transfer cell is operated in a first mode as anelectrochemical hydrogen separator (also referred to as a hydrogenpumping device). Fuel gas exhaust 15 is brought into contact with MEA24. In the example shown in FIG. 3, MEA 24 includes electrolyte 76,electrode layers 78, and gas diffusion layers 80. Power source 26applies a potential across MEA 24, inducing the following reaction ofthe hydrogen in the fuel gas exhaust 15 as it contacts catalyst layer78:

H₂→2H⁺+2e⁻  (3)

[0061] The protons from the reaction flow through the electrolyte 76,and the electrons flow around the MEA 24 to re-form hydrogen accordingto the following reaction:

2H⁺+2e⁻→H₂  (4)

[0062] The relationship of electrical potential between electrodes 27and 29 formed across the cell is described by the Nernst equation:

E=E0 +(RT/nF)*log10(P1/P2)  (5)

[0063] E—measured voltage;

[0064] E0—reactant equilibrium potential

[0065] R—universal gas constant;

[0066] T—temperature;

[0067] n—number of electrons transferred;

[0068] F—Faraday constant;

[0069] P1—hydrogen partial pressure of hydrogen pumping device effluent;and

[0070] P2—partial pressure of hydrogen in the fuel gas exhaust stream.

[0071] Effluent 28 from hydrogen pumping device 24 is substantially purehydrogen because other components of fuel exhaust gas 15 are not passedthrough the MEA 24. The amount of hydrogen transported through MEA 24depends on the amount of current supplied by power source 26. Referringto the direction of hydrogen flow, the MEA 24 has an anode side 27 and acathode side 29. Electrode 27 generally has a positive polarity in thisfunction, whereas electrode 29 generally has a negative polarity.

[0072] This current from power supply 26 provides an exact quantity ofhydrogen transfer from the anode electrode 27 (hydrogen source side) tothe cathode side 29. The hydrogen source for the hydrogen pump anode canbe pure hydrogen or a mixture of hydrogen and other non-reactive gases.A natural gas reformate is an example of a hydrogen source. Reformatemay also be provided from other hydrocarbon sources. Synthesis gas (alsoreferred to as “syn gas”) may also be used as a hydrogen source(referring to a gas contain hydrogen and carbon monoxide), and is acommonly available component of chemical process infrastructure. Thepressure of the source is not significantly important to the operationof the pump except that the voltage may be affected according to theNernst equation (5) (e.g., 0.0295 volt per pressure decade) anddiffusion of the non-reactive gases to the pump cathode will increasewith increased source pressure. The pressure obtained on the hydrogenpump cathode is generally limited only by the structure of the cell.Differential pressures of 5000 psid have been demonstrated in singlecell hardware, however, it is presently preferable to utilize acompressor to pressurize the hydrogen (e.g., into a storage vessel).

[0073] Referring to FIG. 3a, a schematic diagram is shown of a portionof the electrochemical transfer cell of FIG. 2, operated in a secondmode. In this example, the electrochemical transfer cell is operated ina second mode as an electrochemical hydrogen expander. In the exampleshown in FIG. 3a, MEA 24 a includes electrolyte 76 a, electrode layers78 a, and gas diffusion layers 80 a. Hydrogen gas 15 a (e.g., from ahydrogen storage vessel) is brought into contact with MEA 24 a. Hydrogengas 28 a on the low concentration side of MEA 24 a generally has lesshydrogen concentration than the hydrogen gas 15 a.

[0074] The relationship of electrical potential between electrodes 27 aand 29 a formed across the cell is described by the Nernst equation (5).Electrode 27 a generally has a negative polarity in this function,whereas electrode 29 a generally has a positive polarity. Electricalload 26 a draws current between electrodes 27 a and 29 a, inducing thereaction (3) above of the hydrogen in the fuel gas exhaust 15 a as itcontacts catalyst layer 78 a.

[0075] The protons from the reaction flow through the electrolyte 76 a,and the electrons flow around the MEA 24 a to re-form hydrogen accordingto the reaction (4) above.

[0076] Effluent 28 a from hydrogen expander 24 a is substantially purehydrogen. The amount of hydrogen transported through MEA 24 a depends onthe amount of current drawn by load 26 a. Referring to the alternate usein some cases of MEA 24 a as a fuel cell where electrode 29 a is adaptedto receive the oxidant, electrode 29 a is generally referred to as thecathode and electrode 27 a is generally referred to as the anode.

[0077] An example of the system shown in FIG. 3a, the low pressurehydrogen 28 a might be natural gas reformate with a near ambientpressure, with a hydrogen partial pressure of ˜6 psia. The pure highpressure hydrogen 15 a might have a pressure of 600 psia. The cell wouldthen develop a Nernst open circuit voltage of about 0.059 volt, as anexample. At this voltage a resistive load can be applied to the cell andthe flow of hydrogen across the MEA 24 a will be exactly equivalent tothe electrical current generated plus the normal membrane diffusion.Such a system provides various potential advantages over other designs.For example, in general, hydrogen can be more accurately metered withsuch a system, and the power generated by the expander can be used tosupplement the power generation of the fuel cell system. Such a systemmay also be less expensive and more efficient than a turbine expanderthat could be used to generate power from the transfer of the highpressure hydrogen.

[0078] In another example; the pressure of pure hydrogen 15 a might belower, e.g., 25 psia, such that the Nernst potential formed across thecell is too low to use a current draw to transport hydrogen through themembrane 24 a. In such cases, it may be desirable to replace the load 26a with a power supply and operate the cell as a hydrogen pump asdescribed with respect to FIG. 3.

[0079] Referring to FIG. 4, a schematic diagram is shown of a fuel cellsystem 92 with an electrochemical transfer cell 94 according to anotherembodiment of the invention. Fuel cell stack 96 has fuel inlet stream 98and fuel outlet stream 100.

[0080] In some embodiments, a first portion of the fuel outlet stream100 is recirculated into fuel inlet stream 98 through firstrecirculation stream 102. A second portion of fuel outlet stream 100 canbe flowed in a second recirculation stream 104 to electrochemicaltransfer cell 94. Subsystem 94 has a subsystem inlet 106, a subsystemoutlet 108, and a vent 110. When the transfer cell 94 is used as ahydrogen separator, vent 110 disposes of what remains of secondrecirculation stream 104 after it has passed through subsystem 94. Insome embodiments, subsystem 94 can have at least one power supplyingfuel cell 112 and at least one transfer cell 114.

[0081] Subsystem 94 may also have an activation switch 116 connected toelectrical connectors 118 and 120. While power supplying fuel cell 112is part of the fuel cell stack 96, it is electrically separated byelectrical connector 118. In other words, when fuel cell stack 96 is inoperation and switch 116 is closed, the power supplying fuel cell 112generates a voltage potential across electrical connectors 118 and 120.In this way, a potential is provided across hydrogen pumping device 114to induce hydrogen pumping. Where switch 116 is opened, the secondrecirculation stream 104 passes through subsystem 94 and out vent 110without having hydrogen in stream 104 removed by the hydrogen pumpingdevice.

[0082] For example, a voltage of 0.5 VDC across fuel cell 112 may resultin about 7.5 cubic centimeters of hydrogen being “pumped” throughhydrogen pumping device 114 for each amp of current flow.

[0083] Subsystem effluent stream 120 is connected to hydrogen storagedevice 122 and to the fuel inlet stream 98 of the stack 96. Thesubsystem effluent stream 120 and hydrogen storage device 122 may havevarious valve configurations. Hydrogen storage device 122 may be, forexample, a pressure vessel. Where it is desired to charge the pressureof hydrogen storage device 122, the current supplied to fuel cell 112may be selected to produce a sufficient amount of hydrogen to result inthe desired pressure. A compressor may also be used to maintain adesired hydrogen pressure in vessel 122.

[0084] When the transfer cell 94 is used as a hydrogen expander, anelectrical load is placed on cell 94 (e.g., a load that is also normallyplaced on the fuel cell 96), and the cell 94 receives hydrogen fromstorage vessel 122, and expands it into stream 106, as an example, forutilization by the fuel cell 96.

[0085] Referring to FIG. 5, a schematic diagram is shown of a fuel cellsystem with an electrochemical transfer cell operated in a hydrogenseparator mode. A fuel processor 504 receives a flow of natural gas 506and converts it to a hydrogen rich reformate that is flow via hydrogensupply conduit 508 through transfer cell 502, and to fuel cell stack 510via conduit 512. In this example, for simplicity, the air flow to thefuel cell stack 510 is not shown. A portion of the hydrogen in thereformate is reacted in the fuel cell stack and the unreacted portion ofthe reformate is exhausted via conduit 514 to fuel processor 504. Inthis example, a portion of the reformate exhaust 514 is recirculatedthrough the reformer 504, and the remaining reformate exhaust fromconduit 514 is exhausted from the system via conduit 516 (e.g., to anoxidizer). 6The discussion associated with FIG. 3 illustrates the theoryof operation of electrochemical transfer cell 502. Power supply 518causes hydrogen to be pumped from a reformate side 520 of transfer cellMEA 522 to an opposite side 524. The hydrogen is exhausted from the cell502 via conduit 526 to a compressor 528 that charges hydrogen storagevessel 530 at a desired pressure. In this example, the pressure is about5000 psia. Other pressure configurations may also be implemented.

[0086] The storage vessel 530 has an outlet 532 coupled to valve 534that can be used to supply hydrogen from the storage vessel 530 to anexternal application. One such application can include a hydrogen tankassociated with a hydrogen powered vehicle. In such a configuration, thesystem 500 can be a residential fuel cell system, for example, toprovide power to a residence. A hydrogen powered car (e.g., the hydrogenbe used by the vehicle's propulsion system) can be fueled with hydrogenfrom valve 534.

[0087] Systems and methods under the present invention may also be usedin combined heat and power (CHP) systems. For example, the system mayinclude a coolant circuit adapted to transfer heat from the system to aheat sink. Heat may be removed during operation from the fuel cell, thereformer, an exhaust gas oxidizer, or other components that may bepresent. A heat sink may include, as examples, a industrial and potablehot water tanks, heat exchangers for external applications, air heatingsystems for homes or buildings, etc.

[0088] In this example, the storage vessel 530 also includes an outletconduit 536 connected to pressure regulator 538, which leads to valve540, which in this example feeds back into transfer cell 502 when thevalve 540 is open. Pressure regulator 538 can be adjusted to provide adesired pressure from storage vessel 530.

[0089] As an example, the system 500 may include a controller (notshown) to automate the operation of the system 500. The controller maycause the system 500 to operate with the electrochemical transfer cell602 in the hydrogen separator mode when there is a surplus of hydrogenfrom the fuel processor 504. For example, the controller may monitor aload on the fuel cell stack 510, and activate the power supply 518 whenthe load is below a predetermined threshold with respect to the amountof hydrogen produced by the fuel processor 504 (e.g., when the fuelprocessor is producing in excess of 120% of the stoichiometric amount ofhydrogen required by the load). The transfer cell may also have a deadband in which it is not operated, either because the load on the stack510 is appropriate for the amount of hydrogen produced by the fuelprocessor 504, or because the hydrogen storage vessel 530 is full.

[0090] Referring to FIG. 6, a schematic diagram is shown of a fuel cellsystem 600 with an electrochemical transfer cell 602 operated in ahydrogen expander mode. The system 600 is otherwise similar to thesystem discussed with respect to FIG. 5. For example, a systemcontroller as discussed with respect to FIG. 5 may cause the load 618 tobe activated on the transfer cell 602 to initiate the hydrogen expandermode of operation to supply additional hydrogen to fuel cell stack 610(e.g., when the stoichiometric ratio of hydrogen provided to fuel cellstack 610 is less than 120% with respect to an electrical load on thestack. The discussion associated with FIG. 4 illustrates the theory ofoperation of electrochemical transfer cell 602.

[0091] In this mode of operation, valve 640 is opened to allow hydrogenfrom storage vessel 630 to enter side 624 of transfer cell 602. In thisexample, the pressure regulator is set to provide the cell 602 with 1000psia of hydrogen. The hydrogen is transferred through MEA 622 to side620 of transfer cell 602, where the hydrogen is fed to fuel cell stack610.

[0092] While FIG. 6 has been described as a separate system from thesystem discussed with respect to FIG. 5, it will be appreciated that insome embodiments, the systems 500 and 600 can be the same set ofcomponents operated in a different configuration. For example, powersupply 518 may represent a first circuit, and load 618 may represent asecond circuit that can be placed on the same transfer cell. In otherembodiments, it may be desirable to provide separate hydrogen separatorand hydrogen expander subsystems (or only one or the other, depending onthe application desired for the system).

[0093] Referring to FIG. 7, a schematic diagram is shown of a fuel cellsystem with an electrochemical transfer cell operated in a hydrogenseparator mode. A fuel processor 704 receives a flow of natural gas 706and converts it to a hydrogen rich reformate that is flow via conduit708 to fuel cell stack 710. In this example, for simplicity, the airflow to the fuel cell stack 710 is not shown. A portion of the hydrogenin the reformate is reacted in the fuel cell stack and the unreactedportion of the reformate is exhausted to a 3-way valve 750 to fuelprocessor 704 via conduit 714. In this example, a portion of the spentreformate 714 is recirculated through the reformer 704, and theremaining portion is exhausted from the system via conduit 716 (e.g., toan oxidizer). The valve 750 is closed with respect to the transfer cell702, such that no reformate is supplied to the transfer cell.

[0094] In the mode of operation shown in FIG. 7, the electrochemicaltransfer cell 702 is operated as a fuel cell to generate power. Thecompressor 728 is generally not operated in this mode. Hydrogen fromstorage vessel 730 may be supplied via valve 740 and pressure regulator738 to the cell 702 as needed (e.g., to satisfy the electric load on thefuel cell). In this example, the cell 702 is dead-headed, meaning thatthe pure hydrogen from storage vessel 730 is reacted in the cell withoutflowing through the cell (i.e., valve 752 is closed). The pure hydrogengas input into the dual-use PEM stack 702 operating in the fuel cellmode may not have to be humidified if the peak load requirements aresufficiently limited that satisfactory lifetime will still be obtainedsince the cell 702 is only periodically operated.

[0095] Referring to FIG. 8, a schematic diagram is shown of a fuel cellsystem 800 similar to that discussed with respect to FIG. 7, but whereinthe electrochemical transfer cell 802 is operated in a hydrogenseparator mode. A power supply (not shown) is connected to the transfercell 802. Valve 850 is opened such that spent reformate from fuel cell810 is fed to the transfer cell 802. Valve 850 is closed such that noneof the spent reformate is bypassed to the reformer 804.

[0096] Residual hydrogen in the spent reformate is thus transferred viacell 802 to the hydrogen compressor 828, where it is fed to hydrogenstorage vessel 830. Valve 840 is closed in this mode of operation toprevent hydrogen from vessel 830 from entering cell 802.

[0097] The storage vessel 830 has an outlet 832 coupled to valve 834that can be used to supply hydrogen from the storage vessel 830 to anexternal application. One such application can include a hydrogen tankassociated with a hydrogen powered vehicle. In such a configuration, thesystem 800 can be a residential fuel cell system, for example, toprovide power to a residence. A hydrogen powered car (e.g., the hydrogenbe used by the vehicle's propulsion system) can be fueled with hydrogenfrom valve 834. In this embodiment, the electrochemical transfer cell802 refers to a cell that is used as a supplemental fuel cell in onemode of operation, and as a electrochemical hydrogen separator inanother mode of operation. An electrochemical hydrogen expander is notutilized.

[0098] The reformate catalyst electrodes of the dual-use PEM stack 802is able to use platinum without a ruthenium component because the carbonmonoxide in the reformate is completely converted to carbon dioxidewithin the primary PEM reformate/air fuel cell 810. This allows the airto react alternately on the same catalyst electrodes within the dual-usePEM stack 802 without permanently damaging the electrodes by rutheniumoxidation during the reduction process. The pure hydrogen chamber of thePEM hydrogen pump separator 802 becomes the hydrogen fuel chamber forthe cell 802 when it is operated as a fuel cell. This can eliminate theneed to purge the cell with an inert gas such as nitrogen to avoidexothermic oxidation of hydrogen in the cell 802 when air is introduced.

[0099] In some embodiments, it is generally preferable to remove watervapor from the hydrogen stored in the hydrogen vessel 830 to avoidfreezing or corrosion problems. As an example, it may be desirable tolower the dew point of the hydrogen in the vessel 830 to less than 30°C. below zero. In some embodiments, this may be accomplished by flowingthe hydrogen across a desiccant material (not shown), such as a coupledbetween the electrochemical transfer cell 802 and the hydrogen storagevessel 830 (e.g., upstream from compressor 828). Examples of suitabledesiccant materials include molecular sieves, silica gels, clays, andblends of these materials. Such materials include, for example, X- orY-type zeolites (available from Linde Division of UOP), silica gels(available from Davison Division of W. R. Grace), indicating silica gels(available from IMPAK Corp.), Montmorillonite clays (available fromIMPAK Corp.), calcium oxide, and calcium sulfate.

[0100] In some embodiments, two parallel desiccant conduits (not shown)can be provided between the electrochemical transfer cell 802 and thehydrogen storage vessel. For example, when one desiccant material in oneconduit becomes saturated, the second desiccant material in the secondconduit can be used (e.g., a valve switches the hydrogen flow from onedesiccant conduit to another). It may be desirable to size thealternating parallel desiccant beds such that the cycle frequency isless than the electrical power demand cycle on the fuel cell system towhich they are connected.

[0101] Where the transfer cell 802 or fuel cell 810 relies on polymerelectrolyte membranes that require humidification, it may be desirableto humidify the hydrogen provided from storage vessel to fuel cell 810or transfer cell 802 when it is used as a fuel cell. In someembodiments, humidification can be accomplished by flowing the hydrogenfrom the storage vessel across a saturated desiccant material (heated ifnecessary). For example, the flow from the storage vessel 830 may berouted through a desiccant conduit that has been previously saturatedwith water. In some cases, an electric heater in the desiccant materialcan enhance the water that is transferred from the saturated desiccantto the dry hydrogen. The hydrogen may also be humidified by moreconvention means, such as by combining it with steam or passing itacross a membrane humidifier. In still other embodiments, depending onthe lifetime requirements of the fuel cell 810 or of transfer cell 802,it may be acceptable to use dry hydrogen even though this may result insome degradation of performance.

[0102] It will be appreciated that while the present discussion isgenerally focused on the components that comprise various systems underthe present invention, the invention may also be illustrated in terms ofmethods for operating such components. For example, referring to FIG. 8,such a method might include the following steps: (1) flowing hydrogenfrom a hydrogen supply conduit 808 through a fuel cell 810 to provide anelectric current to a load coupled to the fuel cell; (2) exhaustingunreacted hydrogen from the fuel cell to a hydrogen exhaust conduit 860;(3) actuating an electrochemical hydrogen separator 802 in a first modeof operation to transfer hydrogen from the hydrogen exhaust conduit 860to a hydrogen storage vessel 830 (e.g., via conduit 826); and (4)actuating an electrochemical hydrogen expander (see, e.g., FIGS. 3a and6) in a second mode of operation of the system to transfer hydrogen fromthe hydrogen storage 830 vessel to the fuel cell 810.

[0103] Such operating methods associated with the systems describedherein may include and of the features or aspects discussed herein. Forexample, such methods may include any of the following additional steps,either alone or in combination: (a) operating a compressor to pressurizethe hydrogen storage vessel with hydrogen from the electrochemicalhydrogen separator; (b) transferring hydrogen from the hydrogen storagevessel to a secondary hydrogen storage vessel, such as a vehiclepropulsion system; (c) monitoring the load coupled to the fuel cell,performing the step of actuating an electrochemical hydrogen separatorwhen the load is below a first predetermined threshold, and performingthe step of actuating an electrochemical hydrogen expander when the loadis above a second predetermined threshold; (d) flowing hydrogen from theelectrochemical hydrogen expander to a reforming reactor in the secondmode of operation; (e) flowing hydrogen from the electrochemicalhydrogen expander to the hydrogen supply conduit in the second mode ofoperation; (f) flowing the hydrogen from the electrochemical hydrogenseparator across a desiccant; (g) humidifying the hydrogen transferredfrom the hydrogen storage vessel in the second mode of operation; and(h) flowing the hydrogen from the electrochemical hydrogen expanderthrough a humidifier. Other methods and steps may also be provided.

[0104] While the invention has been disclosed with respect to a limitednumber of embodiments, those skilled in the art, having the benefit ofthis disclosure will appreciate numerous modifications and variationstherefrom. It is intended that the invention covers all suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method of operating a fuel cell system,comprising: placing an electrical potential across a first fuel cell ina first mode of operation to transfer hydrogen from an anode chamber ofthe first fuel cell to a cathode chamber of the first fuel cell; placingan electrical load across the first fuel cell in a second mode ofoperation to transfer hydrogen from the cathode chamber of the firstfuel cell to the anode chamber of the first fuel cell; and flowing airthrough the anode chamber of the first fuel cell and hydrogen throughthe cathode chamber of the first fuel cell in a third mode of operationto provide an electric current to a load coupled to the first fuel cell.2. The method of claim 1, further comprising: supplying reformate from areforming reactor to the anode chamber.
 3. The method of claim 1,further comprising: operating a compressor in the first mode ofoperation to pressurize a hydrogen storage vessel with the hydrogentransferred from the anode chamber to the cathode chamber.
 4. The methodof claim 1, further comprising: transferring hydrogen in the first modeof operation from the cathode chamber to a hydrogen storage vessel; andtransferring hydrogen from the hydrogen storage vessel to a secondaryhydrogen storage vessel.
 5. The method of claim 4, wherein the secondaryhydrogen storage vessel is a portion of a vehicle propulsion system. 6.The method of claim 1, further comprising: monitoring an electrical loadon a second fuel cell; switching the system to the first mode ofoperation when the load is below a first predetermined threshold; andswitching the system to the second mode of operation when the load isabove a second predetermined threshold.
 7. The method of claim 1,further comprising: flowing hydrogen from the anode chamber of the firstfuel cell to a reforming reactor in the second mode of operation.
 8. Themethod of claim 1, further comprising: flowing hydrogen from the anodechamber of the first fuel cell to a second fuel cell in the second modeof operation.
 9. The method of claim 1, in the first mode of operation,further comprising: flowing the hydrogen from the cathode chamber of thefirst fuel cell across a desiccant.
 10. The method of claim 1, in thesecond mode of operation, further comprising: flowing the hydrogen fromthe anode chamber of the first fuel cell through a humidifier.
 11. Amethod of operating a fuel cell system, comprising: flowing hydrogenfrom a hydrogen supply conduit through a fuel cell to provide anelectric current to a load coupled to the fuel cell; actuating anelectrochemical hydrogen separator in a first mode of operation of thesystem to transfer hydrogen from the hydrogen supply conduit to ahydrogen storage vessel; and actuating an electrochemical hydrogenexpander in a second mode of operation of the system to transferhydrogen from the hydrogen storage vessel to the fuel cell.
 12. Themethod of claim 11, further comprising: supplying reformate from areforming reactor to the hydrogen supply conduit.
 13. The method ofclaim 11, further comprising: operating a compressor to pressurize thehydrogen storage vessel with hydrogen from the electrochemical hydrogenseparator.
 14. The method of claim 11, further comprising: transferringhydrogen from the hydrogen storage vessel to a secondary hydrogenstorage vessel.
 15. The method of claim 11, further comprising:monitoring the load coupled to the fuel cell; performing the step ofactuating an electrochemical hydrogen separator when the load is below afirst predetermined threshold; and performing the step of actuating anelectrochemical hydrogen expander when the load is above a secondpredetermined threshold.
 16. The method of claim 11, further comprising:flowing hydrogen from the electrochemical hydrogen expander to areforming reactor in the second mode of operation.
 17. The method ofclaim 11, further comprising: flowing hydrogen from the electrochemicalhydrogen expander to the hydrogen supply conduit in the second mode ofoperation.
 18. The method of claim 11, further comprising: flowing thehydrogen from the electrochemical hydrogen separator across a desiccant.19. The method of claim 11, further comprising: humidifying the hydrogentransferred from the hydrogen storage vessel in the second mode ofoperation.
 20. A method of operating a fuel cell system, comprising:flowing hydrogen from a hydrogen supply conduit through a fuel cell toprovide an electric current to a load coupled to the fuel cell;exhausting unreacted hydrogen from the fuel cell to a hydrogen exhaustconduit; actuating an electrochemical hydrogen separator in a first modeof operation of the system to transfer hydrogen from the hydrogenexhaust conduit to a hydrogen storage vessel; and actuating anelectrochemical hydrogen expander in a second mode of operation of thesystem to transfer hydrogen from the hydrogen storage vessel to the fuelcell.
 21. The method of claim 20, further comprising: supplyingreformate from a reforming reactor to the hydrogen supply conduit. 22.The method of claim 20, further comprising: operating a compressor topressurize the hydrogen storage vessel with hydrogen from theelectrochemical hydrogen separator.
 23. The method of claim 20, furthercomprising: monitoring the load coupled to the fuel cell; performing thestep of actuating an electrochemical hydrogen separator when the load isbelow a first predetermined threshold; and performing the step ofactuating an electrochemical hydrogen expander when the load is above asecond predetermined threshold.
 24. The method of claim 20, furthercomprising: flowing hydrogen from the electrochemical hydrogen expanderto a reforming reactor in the second mode of operation.
 25. The methodof claim 20, further comprising: flowing hydrogen from theelectrochemical hydrogen expander to the hydrogen supply conduit in thesecond mode of operation.
 26. The method of claim 20, furthercomprising: flowing the hydrogen from the electrochemical hydrogenseparator across a desiccant.
 27. The method of claim 20, furthercomprising: flowing the hydrogen from the electrochemical hydrogenexpander through a humidifier.